More millionaires and fewer large corporations in the U.S. face tax audits because of increases in the share of income earned through partnerships and the creation of a special audit group in 2009, a Bloomberg study found.

Partnerships and other organizations that pass income through to the individual have become increasingly common.

Partnerships accounted for 21 percent of business income in 2004, up from 3 percent in 1990, according to a 2007 study from the Treasury Department. Partnership income is concentrated in high-income individual tax returns.

The Internal Revenue Service audited 8.4 percent of individuals with reported income of more than $1 million last year, the highest rate since the agency began publishing that statistic in 2004. The rate was 18.4 percent for those reporting more than $10 million, up from 10.6 percent in 2009. The audit rate for corporations with $250 million or more in assets fell to 23.4 percent last year, its lowest rate since an IRS restructuring in the late 1990s.

The data suggest that wealthy individuals who move their income to flow-through entities from corporations to avoid the corporate-level tax might increase the likelihood of being audited.

The IRS established a Global High Wealth Industry group in 2009 as part of the Large Business and International Division. Audits of high-income individuals and corporations are handled within that division.